Case hardened alloy steels



Patented Nov. 13, 1934 CASE HARDENED ALLOY STEELS Ernest C. Mofiett, Woodbridge, N. .L, assignor to American Cyanamid Company, New York,

N. Y., a corporation of Maine No Drawing. Application February 12, 1930,

, Serial No. 427,943

20mins.

This invention relates to case hardened alloy steels and is concerned more particularly with alloy steels which have been treated in a case hardening bath containing an alkali earth metal cyanide.

In the past attempts to case harden alloy steel, using the known methodssuch as pack carburizing or the usual case hardening baths, have not met with the desired success. These methods result in lack of uniformity, case of insuflicient depth and/or lack of ductility.

. In accordance with my invention, I have found it commercially feasible to harden the various types of alloy steels, producing a case of considerable depth, by treating them in a case hardening bath containing an alkali earth metal cyanide such as calcium cyanide. As described more fully in my co-pending application, Serial #209,644, filed July 30, 1927 now Patent No.

0 1,796,800, this case hardening bath is preferably made up of a mixture of two parts by weight calcium chloride and one part sodium chloride. This mixture is melted and to it is added about 1% calcium cyanide- Articles to be hardened are immersed in the bath for the desired period of time, while maintaining the temperature of the bath at the desired point. The treated articles are removed from the bath at the expiration of this period of time, are quenched and may be submitted to a drawing operation, if desired. Howeverythis latter operation is not especially necessary in contradistinction to the usual methods.

More specifically, an alloy steel having the following composition was subjected to the described case hardening operation, the figures given being percentages by weight:

0 Mn 5 P Si Cr 491 .68 024 .Ofl) 15 1.07

The following tabulation indicates the conditions of treatment and the results obtained:-

The case produced ranged from 0.006" at 1420" F. to 0.009" at 1510 F. The Rockwell hardness, C scale, averaged 57.0 and the Izod impactvalues for 36 tests averaged 2.91 foot pounds.

The following data is the summary of results. obtained using the same steel in a sodium cyanide bath, under conditionsspecified.

From the above data it will be apparent that the materials treated in the calcium cyanide bath are harder than those treated by sodium cyanide, and with an impact value more than 50% greater. The superiority of the case is still 0 more forcibly shown by a comparison of the relative scleroscope hardness values. It is also to be noted that a greater depth of case is produced with calcium cyanide. In view of the deeper penetration of carbon produced, it is possible to use a lower carbon steel for many purposes requiring a higher carbon'steel with the resulting economy in fabrication.

Likewise, with an S. A. E. 6150 chrome-vanadium steel, treated in a calcium cyanide bath, the material had a hardness of 6466 on the Rockwell C scale, whereas similar material treated in the sodium cyanide bath had a hardness of 54-57.

A number of dies, made of S. A. E. 3250 nickelchromium steel, were treated in the calcium cyanide bath and were found to give seven to ten hours service. These steel dies heat treated in the usual manner last less than one hour. Even the best tool steels, heat treated according to standard practice, give but from 1 to '6 hours service.

A further advantage of the hardened steels forming a part of the, present invention is the fact that they are uniform and possess high ductility. From examination of the above figures, it will be seen that not only the life of niy steels is high but they vary to only a slight degree. In the usual alloy steels, an occasional product may have a high life but the variation is quite considerable.

nickel steel were pack carburized and the first 0.0 05" cut from the case, upon analysis, was found to contain 0.90-l.00% carbon. Further treatment of the carburized steel in a case hardening bath containing calcium cyanide, produced a steel in which the first cut from the case contained 1.35-1.44% carbon. Other samples of the carburized steel which were treated in a sodium cyanide case hardening bath were analyzed and the first cut from the case was found to contain less than 0.90% carbon. From this it will be seen that my treatment increases the'carbon content of the case produced by pack carburization. This has been impossible of accomplishment heretofore and, in fact, the sodium cyanide treatment extracts some of the carbon.

One of the most important advantages of my invention is the ability to produce a case having a very high carbon content, much higher than that produced by any other previously known method. For example, I am able to obtain a carbon content of 1.5 per cent by treatment of alloy steel in a calcium cyanide case hardening bath for 30 minutes or less.

When sodium cyanide is used as a case hardening material, it is virtually impossible to obtain a carbon content greater than 0.7 per cent no matter how long the heating takes place.

Prior to the present invention pack carburizing was used almost exclusively for case hardening alloy steels and for this purpose it was necessary to treat the material for from 5 to 15 hours'an'd the carbon content of the case does not exceed 1 per cent no matter how long the case hardening operation is continued. My process, therefore, gives a result which was commercially impractical by use ofprior methods. Why my process results in this improvement I cannot explain except that I have proven by large scale operations that as a, matter of fact high carbon contents are obtained in a short time of treatment.

' Because of the amount of carbon in the case, the concentration of the cementite is much greater than in articles case hardened by the 01d methods, so that upon quenching or other suitable treatment, the structure of the steel becomes harder. The short time of case hardening and hence the short period of heat treatment gives the desired case and prevents any material amount of grain growth. Because of the compactand small crystalline structure the case is considerably superior to that produced by pack carburizing. f

I have found that it is possible to case harden all types of alloy steels including the following:

Nickel steels,

Nickel-chromium steels, Molybdenum steels, -Chromium steels,

Chrome-vanadium steels, Tungsten steels, Silico-manganese steels, Nickel-molybdenum steels, and High manganese steels.

It will be obvious that many variations may be introduced in the method-0f case hardening utilized for carrying out my invention. The nature of the alkali earth metal cyanide, the composition of the hardening bath, the temperatures and times of treatment, etc., .may be changed in accordance with the results desired, without departing from the spirit and scope of the invention.

1. An alloy steel having less than 1 per cent of carbon in the body of the steel and having a carburized surface containing more than 1 per cent of carbon, the steel having a fine grain structure, which carburized surface is obtained an alkaline earth metal cyanide.

2. An alloy steel having lessthan 1 per cent of carbon in the body of the steel and having a carburized surface containing more than 1.35 per cent of carbon, the steel having a fine grain structure, which carburized surface is obtained by hardening in a molten saltb'ath containing an alkaline earth metal cyanide.

ERNEST C. MOFFETT. 

